1. Field of the Invention
This invention relates to the design and fabrication of magnetic tunnel junctions (MTJ) as memory storage devices, particularly to a design wherein coercivity and anisotropy are decoupled from the cell shape of the junction and can be independently optimized.
2. Description of the Related Art
The magnetic tunnel junction (MTJ) basically comprises two electrodes, which are layers of ferromagnetic material, separated by a tunnel barrier layer, which is a thin layer of insulating material. The tunnel barrier layer must be sufficiently thin so that there is a probability for charge carriers (typically electrons) to cross the layer by means of quantum mechanical tunneling. The tunneling probability is spin dependent, however, depending on the availability of tunneling states with different electron spin orientations. Thus, the overall tunneling current will depend on the number of spin-up vs. spin-down electrons, which in turn depends on the orientation of the electron spin relative to the magnetization direction of the ferromagnetic layers. Thus, if these magnetization directions are varied for a given applied voltage, the tunneling current will also vary as a function of the relative directions. As a result of the behavior of an MTJ, sensing the change of tunneling current for a fixed potential can enable a determination of the relative magnetization directions of the two ferromagnetic layers that comprise it. Equivalently, the resistance of the MTJ can be measured, since different relative magnetization directions will produce different resistances.
The use of an MTJ as an information storage device requires that the magnetization of at least one of its ferromagnetic layers can be varied relative to the other and also that changes in the relative directions can be sensed by means of variations in the tunneling current or, equivalently, the junction resistance. In its simplest form as a two state memory storage device, the MTJ need only be capable of having its magnetizations put into parallel or antiparallel configurations (writing) and that these two configurations can be sensed by tunneling current variations or resistance variations (reading). In practice, the free ferromagnetic layer can be modeled as having a magnetization which is free to rotate but which energetically prefers to align in either direction along its easy axis (the direction of magnetic crystalline anisotropy). The magnetization of the fixed layer may be thought of as being permanently aligned in its easy axis direction. When the free layer is anti-aligned with the fixed layer, the junction will have its maximum resistance, when the free layer is aligned with the fixed layer, the minimum resistance is present. In typical MRAM circuitry, the MTJ devices are located at the intersection of current carrying lines called word lines and bit lines (or word lines and sense lines). When both lines are activated, the device is written upon, ie, the magnetization direction of its free layer is changed. When only one line is activated, the resistance of the device can be sensed, so the device is effectively read. Such an MTJ device is provided by Gallagher et al. (U.S. Pat. No. 5,650,958), who teach the formation of an MTJ device with a pinned ferromagnetic layer whose magnetization is in the plane of the layer but not free to rotate, together with a free magnetic layer whose magnetization is free to rotate relative to that of the pinned layer, wherein the two layers are separated by an insulating tunnel barrier layer.
In order for the MTJ MRAM device to be competitive with other forms of DRAM, it is necessary that the MTJ be made very small, typically of sub-micron dimension. Such a small area cell is provided by Gallagher et al. (U.S. Pat. No. 6,226,160 B1) who make use of a tunnel barrier layer formed of an oxidized thin aluminum layer. Parkin et al. (U.S. Pat. No. 6,166,948) notes that sub-micron dimensions are needed to be competitive with DRAM memories in the range of 10–100 Mbit capacities. Parkin also notes that such small sizes are associated with significant problems, particularly super-paramagnetism, which is the spontaneous thermal fluctuation of magnetization in samples of ferromagnetic material too small to have sufficient magnetic anisotropy (a measure of the ability of a sample to maintain a given magnetization direction). It is also undesirable for MTJ devices to have excessive magnetic coupling between adjacent magnetic layers of neighboring devices or even within the same device as this coupling must be overcome when writing on the device. One source of such undesirable coupling results from the non-planar surfaces at the interfaces of ferromagnetic layers, such as might occur between the fixed and free layer of an MTJ. This is known as topological coupling. Slaughter et al. (U.S. Pat. No. 6,205,052 B1) teaches a way of reducing such topological coupling by forming an additional layer between a base metal and a spacer layer, the additional layer being crystallographically amorphous with respect to x-ray scattering analysis.
Some degree of anisotropy is necessary if an MTJ cell is to be capable of maintaining a magnetization direction and, thereby, to effectively store data even when write currents are zero. As cell sizes have continued to decrease, the technology has sought to provide a degree of magnetic anisotropy by forming cells in a wide variety of shapes (eg. rectangles, diamonds, ellipses, etc.), so that the lack of inherent crystalline anisotropy is countered by a shape anisotropy. Yet this form of anisotropy brings with it its own problems. A particularly troublesome shape-related problem in MTJ devices results from non-uniform and uncontrollable edge-fields produced by shape-anisotropy (a property of non-circular samples). As the cell size decreases, these edge fields become relatively more important than the magnetization of the body of the cell and have an adverse effect on the storage and reading of data. Although such shape anisotropies, when of sufficient magnitude, reduce the disadvantageous effects of super-paramagnetism, they have the negative effect of requiring high currents to change the magnetization direction of the MTJ for the purpose of storing data. To counteract these edge effects, Shi et al. (U.S. Pat. No. 5,757,695) teaches the formation of an ellipsoidal MTJ cell wherein the magnetization vectors are aligned along the length (major axis) of the cell and which do not present variously oriented edge domains, high fields and poles at the ends of the element. In addition, the fabrication processes required to produce the shape varieties, eg. photolithography and ion-milling, are incapable of controlling the shaping with sufficient precision to prevent variations in cell sizes, shapes and aspect ratios and, in addition, cause uncontrollable defects along the edges of the cells. The randomness of all these defects lead to a wide distribution in switching field coercivities (the fields required to change the logic state of an MTJ cell) and can even cause unwanted and uncontrollable coupling between cells. One attempt to reduce edge effects is provided by Nakao et al. (U.S. Pat. No. 6,351,410 B1), who form ring shaped MTJ electrodes to cause the induced magnetic fields to be circumferential.
As has been discussed, many of the problems associated with the construction of MRAM arrays are related to the shapes of the cells and the processes required to form those shapes. Cell shapes of present designs are typically single element rectangle, elliptical or lozenge. Any irregularities of these shapes, or defects at their edges produced during their formation, will result in coercivity fluctuations distributed throughout the array. An alternative approach to providing magnetic anisotropies without the necessity of utilizing shapes which are difficult to fabricate, is to produce a magnetic anisotropy in a ferromagnetic layer by forming it on an antiferromagnetic layer. In this way, a magnetic coupling can be produced between the ferromagnetic and antiferromagnetic layers which will provide the required magnetic anisotropy. In “Orientational dependence of the exchange biasing in molecular-beam-epitaxy-grown Ni80Fe20/Fe50Mn50 bilayers” (R. Jungblut, R. Coehoorn, M. T. Johnson, J. aan de Stegge and A. Reinders, J. Appl. Phys. 75(10), 15 May 1994, pp. 6659–6664), experimental results are provided to show that interfacial exchange energy between such layers can be utilized to provide a biasing effect which lowers coercivity (as indicated by hysteresis loop shifts) in crystal growth directions. Fujikata et al. (U.S. Pat. No. 5,766,743) provide a magnetoresistance effect film having two ferromagnetic layers separated by a non-magnetic layer wherein one of the ferromagnetic layers is formed on an antiferromagnetic layer. The antiferromagnetic layer is at least partly made of a NiMn alloy having a face-centered tetragonal structure and it provides a biasing magnetic field, Hr, which exceeds the coercive force, Hc2, of the other ferromagnetic layer. The purpose of the antiferromagnetic layer is to provide domain stabilization of the free layer and, as a result the antiferromagnetic layer is not formed over the active region of the free layer. In a related invention, Rizzo et al. (U.S. Pat. No. 6,430,084 B1) teach the formation of bit and digit lines (the lines whose currents write and read the MTJ devices) which are clad with shielding ferromagnetic and antiferromagnetic layers to prevent inadvertent switching of adjacent MTJ cells. In the structure provided by Rizzo, the antiferromagnetic layer stabilizes the magnetization of the ferromagnetic layer by exchange coupling and thereby improves the shielding effect. It is the object of the present invention to utilize the magnetic coupling properties of antiferromagnetic layers with ferromagnetic layers to provide the necessary magnetic anisotropy to form an MTJ memory cell capable of advantageously storing data without the necessity of obtaining the anisotropy through the route of shape anisotropy.